Light emitting devices of optical semiconductor devices such as light emitting diodes (LEDs) are usually sealed with a sealing agent to avoid direct exposure to the air because their light emitting properties rapidly deteriorate due to water, floating dust and the like in the air. Epoxy resins such as bisphenol type epoxy resins and alicyclic epoxy resins have been used as a resin component in sealing agents for sealing these light emitting devices (see, for example, Patent Document 1) as these resins provide strong adhesion and have excellent dynamic durability.
However, these conventional sealing agents containing an epoxy resin have insufficient heat resistance and light resistance to seal high brightness optical semiconductor devices that have been intensively studied in recent years, and thereby cured products thereof may be problematically colored.
There are some studies to overcome this problem including a study on addition of a compound to improve the heat resistance of an epoxy resin (see, for example, Patent Document 2). However, neither of them achieves sufficient heat resistance.
In addition, a method is known in which a highly heat-resistant and light-resistant silicone resin with high transmittance of light of a short wavelength in the blue to ultraviolet region is used in a sealing agent for sealing an LED light emitting device instead of the epoxy resins.
However, since foreign matters tend to adhere to the light emitting surface owing to surface tackiness of the silicone resin sealing agent, use of the silicone resin sealing agent problematically tends to cause damage to the light emitting surface of the light emitting device. Such damage to the light emitting surface may lead to cracks developing therefrom or a remarkable decrease in the brightness of the light emitting device.
In order to overcome this problem, methods using a silicone resin sealing agent with high crosslink density have been studied. The silicone resin sealing agent with a high crosslink density has no surface tackiness and thereby can prevent adhesion of foreign matters and damage to the light emitting surface, but has significantly low mechanical strength and provides very poor adhesion. If the silicone resin sealing agent is repeatedly exposed to a heat cycle, cracks will form in the sealing agent, or the sealing agent will be peeled off from a housing material etc. Because of high moisture permeability of the silicone resin, the light emitting properties of the light emitting device may deteriorate through prolonged use. In addition, as the silicone resin is less refractive, an optical semiconductor having a light emitting device sealed with the silicone resin provides insufficient light extraction efficiency.
Although methods in which a silicone resin and an epoxy resin are used in combination to cover the disadvantages of each resin have been studied to overcome these problems, sufficient performance has not been achieved yet.
The light emitting devices are sealed with a sealing agent, for example, by a method comprising: filling a frame having light emitting devices, etc. disposed therein with the sealing agent without leaving any space around the light emitting devices; and curing the sealing agent. The conventional sealing agents cannot stably maintain their shape when charged into the frame, and thereby cured products of the sealing agent sealing the light emitting device problematically show variations in the shape.
Recently, application of the optical semiconductor devices, for example, to backlights for liquid crystals, headlights for vehicles, and general lighting instruments has been examined. In such application, a sealing agent containing a yellow phosphor is used with a blue light emitting device to generate white light (see, for example, Patent Documents 3 and 4). A sealing agent containing a yellow phosphor and a red phosphor is also used to improve color rendering properties. Various combinations of a light emitting device and a phosphor have been actively studied to generate white light.
However, these sealing agents containing a phosphor show deterioration of the viscosity when being cured by heating, and thereby may cause sedimentation of the phosphor therein. As a result, the color of light to be generated may be problematically non-uniform. The problem of non-uniform color is fatal, particularly for lighting use.
For example, a sealing agent prepared to have high viscosity at room temperature may avoid deterioration of the viscosity caused by heating, and thereby prevent sedimentation of the phosphor.
However, such a sealing agent having high viscosity at room temperature causes problems such as incomplete sealing, variations in the discharge amount of the sealing agent, elongation of the sealing process because the flowability of the sealing agent is insufficient for sealing.    Patent Document 1: Japanese Kokai Publication 2003-277473 (JP-A 2003-277473)    Patent Document 2: Japanese Kokai Publication 2003-73452 (JP-A 2003-73452)    Patent Document 3: Japanese Kokai Publication 2002-314142 (JP-A 2002-314142)    Patent Document 4: Japanese Kokai Publication 2004-127988 (JP-A 2004-127988)